1. Field of the Invention
The present invention relates to a microstrip antenna. In particular, the present invention relates to a microstrip antenna which can minimize leakage current by separately arraying a left radiation patch and a right radiation patch on an upper surface of a dielectric so that they have an electric field of the same phase, and which can minimize its size and thus can be built in various kinds of wireless communication equipment such as portable mobile terminals by improving its standing-wave ratio and gain so that it has a wide bandwidth.
2. Description of the Prior Art
Generally, frequencies mainly used in mobile radio communications are in the range of 150xcx9c900 MHz. Recently, according to the rapidly increasing demand therefore, frequencies of a pseudo-microwave band in the range of 1xcx9c3 GHz are also used.
In applying the pseudo-microwave band to the mobile radio communications, personal communication service (PCS) has already used a frequency range of 1.7xcx9c1.8 GHz, and next-generation mobile radio communication systems such as GMPCS (1.6 GHz), IMT2000 (2 GHz), etc., will also use the pseudo-microwave band to enable communications through all places of the world.
As portable telephones become small-sized and high-graded by their rapid development, the importance of their antennas have been naturally highlighted, and as an example, a microstrip antenna has been presented as the subject of special research in this field.
Typically, the microstrip antenna has a better efficiency as a dielectric constant becomes lower, and a substrate becomes thicker. Also, since the microstrip antenna has a low efficiency in case of using a low frequency, but has a high efficiency in case of using a high frequency, it can be considered as the very antenna that can satisfy the limited condition of miniaturization that the portable telephone pursues.
Meanwhile, a typical microstrip antenna has a structure in which radiation patches having a resonance length of xcex/2 are attached on a wide ground patch, and has the form of an array. Between the patches on the left and right sides of a feed point and the ground patch are formed lines of electric force. If the ground patch is short on the left and right sides of the feed point, this limits the formation of the lines of electric force, and thus lowers the gain of the antenna, causing the of miniaturization of the antenna to be difficult.
The microstrip antenna has a simple structure in which a dielectric is formed on the ground patch, and rectangular or circular radiation patches are attached on the upper surface of the dielectric, and thus it has drawbacks in that it has a narrow bandwidth and a low efficiency. However, it has advantages in that it can be manufactured at a low cost with a small size and a light weight, and thus it is suitable to mass production.
Also, since it can be wound on various devices and components with a predetermined form due to its free banding characteristic and can be easily attached to an object moving at a high speed, it has been widely used as a transmission/reception antenna of a flying object such as a rocket, missile, airplane, etc.
In addition, the microstrip antenna can be designed on a circuit board together with solid-state modules such as an oscillator, amplifying circuit, variable attenuator, switch, modulator, mixer, phase shifter, etc.
The microstrip antenna as described above may be designed so as to have one or two feed points and circular or rectangular radiation patches in a satellite communication system that requires circularly polarized waves. Also, it can be used for a Doppler radar, radio altimeter, remote missile measuring device, weapon, environmental machine and its remote sensor, transmission element of a composite antenna, remote control receiver, radiator for biomedicine, etc.
As a result, with the rapid spread of mobile communication terminals such as telephones for vehicles, pocket bells, cordless telephones, etc., due to the rapid development of information processing, the equipment for such mobile communications becomes small-sized, and this demands that the antenna thereof also to become small-sized.
FIG. 1 is a side view illustrating a general microstrip antenna. Referring to FIG. 1, the general microstrip antenna has a radiation patch 1 both ends of which are open, and thus the current distribution of which is 0 and the voltage distribution of which is a maximum value. A feed position is determined as the ratio of the current distribution value to the voltage distribution value in accordance with the resistance value of a feed line 2.
Also, lines of electric force, 3 and 5, can be considered to be divided into a vertical component and a horizontal component, respectively. The vertical components are cancelled due to their opposite phase to each other, and the horizontal components exist in array due to their same phase.
If the length of the ground patch 6 in the microstrip antenna is determined to be short, the range where the lines of electric force, 3 and 5, exert is limited, and this results in attenuation of the gain. Thus, shortening the ground patch 6 cannot achieve the miniaturization of the antenna.
Generally, the microstrip antenna is a unit of a VHF/UHF band, and is required to have a compact and light-weighted structure. As the presently developed microstrip antenna, a quarter-wavelength microstrip antenna (QMSA), post-loading microstrip antenna (PMSA), window-attached microstrip antenna (WMSA), frequency-variable microstrip antenna (FVMSA), etc., exist. The PMSA, WMSA, and FVMSA are provided by partially modifying the QMSA, and thus basically have similar radiation patterns to one another.
FIG. 2 is a perspective view illustrating the structure of a conventional QMSA. Referring to FIG. 2, according to the conventional QMSA, a radiation patch 23 and a ground patch 21 are constructed so that they have an identical width W, and the ground patch 21 extends in a direction opposite to a radiation opening 22 to provide a small-sized antenna that can be mounted in a limited space of a small-sized radio device.
Specifically, according to the QMSA of FIG. 2, a dielectric 22 and the radiation patch 23 are successively attached to the ground patch 21 of xcexg (guide wavelength)/2, one end of the ground patch 21 is short-circuited to the radiation patch 23, and the length of the radiation patch 23 is determined to to be xcexg/4 to have a fixed frequency range.
Also, an outer conductor of a feed line 24 is grounded to the ground patch 21, and an inner conductor (center conductor) of the feed line 24 is connected to the radiation patch 23 through the ground patch 21 and the dielectric 22 (Japanese Electronic Information Society, Vol. J71-B, 1988.11.). Typically, polyethylene (xcex5r=2.4), Teflon (xcex5r=2.5), or epoxy-fiberglass (xcex5r=3.7) can be used as the dielectric 22.
FIG. 3 shows the variation of the gain ratio according to the variation of Gz in FIG. 2. In FIG. 2, 0 (dB) represents the gain of a basic half-wavelength dipole antenna. Gz plays a very important role for determining the increasing rate of radiation. FIG. 4 shows the variation rate of gain according to the whole length L of the antenna of FIG. 2, and FIG. 5 shows the gain ratio to the width W of the radiation patch 23 of FIG. 2.
FIG. 6 shows the measured radiation property of the QMSA of FIG. 2. In FIG. 6, (A), (B), (C) represent an XY plane, YZ plane, and ZX plane, respectively. As shown in FIG. 6, it can be recognized that the QMSA of FIG. 2 is an electric field antenna having the radiation patterns in all propagation directions. The radiation characteristics of the QMSA are obtained by determining parameters of the antenna as the whole length L of the antenna=7.67 cm, Gz=2.79 cm, the width W of the radiation patch 23=3 cm, the width t of the dielectric 22=0.12 cm, and dielectric constant xcex5r=2.5 (Teflon).
Meanwhile, when the standing-wave distribution is positioned near its minimum point in a complicated city environment, the transmission/reception sensitivity of the electric field antenna deteriorates due to the diffraction, reflection, etc., of the signal, and this causes the communication to be disturbed.
Accordingly, the current radio equipment or system uses a spatial diversity, directional diversity, polarized diversity, etc. Meanwhile, two or more antennas may be installed to solve the low reception sensitivity caused by a multipath.
Meanwhile, according to the PMSA (not illustrated) which is a modified microstrip antenna, two radiation open surfaces are formed on both sides of a radiation patch, a short-circuited post is connected to a ground patch and the radiation patch through a dielectric instead of a short-circuited end of the QMSA antenna, and a feed line is located at a predetermined distance from the short-circuited post. Though the PMSA has two open surfaces, the radiation pattern thereof is substantially similar to that of the QMSA.
Also, according to the WMSA (not illustrated) which is a modified microstrip antenna, a slit is formed at a predetermined distance from the radiation patch of the QMSA to have a reactance component, and thus the length of the radiation patch can be shortened. According to the FVMSA (not illustrated), the resonance frequency of the QMSA can be electronically changed in accordance with the change of the reactance load value.
However, the conventional modified microstrip antennas, i.e., the QMSA, PMSA, WMSA, and FVMSA have drawbacks in that if the ground patch is determined to be small, the radiation open surfaces become narrow, and their gains are rather attenuated, so that they cannot be small-sized. Also, if they are used for portable radio equipment, the field strength thereof deteriorates due to walls of a building and various metals in the building, and the receiving sensitivity deteriorates due to the multipath interference.
It is an object of the present invention to solve the problems involved in the related art, and to provide a microstrip antenna which can greatly miniaturize its size without attenuation of its gain and without limiting the range of lines of electric force between a ground patch and radiation patches, and which can have a wide bandwidth by implementing a greater gain on a capacity-loaded side rather than the ground patch.
In order to achieve the above object, there is provided a microstrip antenna having a ground patch on which at least a feed line is located, and a dielectric laminated on the ground patch, the microstrip antenna comprising a left radiation patch short-circuited to one end of the ground patch and laminated on a left upper surface of the dielectric, and a right radiation patch short-circuited to the other end of the ground patch and laminated in an array on a right upper surface of the dielectric with a radiation slot arranged between the left and right radiation patches so that capacitance is implemented between the left and right radiation patches, wherein the ground patch includes a right ground plate having an area of a triangle formed by a feed point of a feed line and both corners of a right lower surface of the dielectric to which the right radiation patch is short-circuited, a connection plate having a narrow width and extending as long as a height of the right ground plate from the feed point to the left radiation patch to implement an inductance, and a left ground plate connected to the connection plate and covering a left lower surface of the dielectric.
Preferably, the microstrip antenna according to the present invention further includes a mounting piece having a bent shape and attached to a center portion of a left end of the left radiation patch, one side surface of the dielectric, and the left ground plate to provide a height for enabling the ground patch to be separately mounted.